With the outbreak of World War II, he was mobilized a Captain in the RNVR on 9 September 1939 into the Naval Staff, Operations Division at the Admiralty. There he was in charge of the War Room of the newly appointed First Lord of the Admiralty, Winston Churchill and later headed the Map Room at 10 Downing Street,[2] when Churchill became prime minister. Pim remained at Downing Street during the course of the war except for a brief stint on the Staff Allied Naval Commander, Algiers. Pim attended many overseas trips to set up temporary map rooms for the meetings with Roosevelt and later Stalin.